1. Field of the Invention
The present invention relates to a method for melting incineration residues containing salts, such as an incineration residue from municipal refuse, and an apparatus therefor.
2. Description of the Related Arts
A major portion of incineration residue which forms during incineration from municipal refuse and industrial waste has been disposed in landfills, and volume reduction of the residue has been required as reclaimed grounds can be secured only with great difficulties. Since ash (fly ash) in the incineration residue, which is scattered with exhaust gas after combustion and collected by a dust collector, contains heavy metals, such as lead and cadmium, and is designated as general waste under special control, it must be subjected to innocuous disposal before dumping. Therefore, both volume reduction of the incineration residue in the incinerator and innocuous disposal of scattered ash are required.
Disposal methods for melting and solidifying the incineration residue have been developed as a technology for simultaneously achieving volume reduction of the incineration residue and innocuous disposal of heavy metals by insolubilization in order to overcome the above-mentioned problems. One method for melting and solidifying includes continuous-feeding and melting of the incineration residue while holding the melt of the incineration residue. This method is capable of separating the melt components held in the furnace into a molten slag layer and a molten salt layer by the differences in specific gravity, and thus the melt can be fractionally exhausted as molten slag and molten salt.
In the above-mentioned method, the molten salt is always present in the furnace, and large amounts of low-boiling-point materials, i.e. alkaline chlorides, such as NaCl and KCl, which are contained in the molten salt, are volatilized. The volatilized alkaline metal salts condense in the components of the exhausting system, result in clogging the exhausting system, corroding the components, and inhibit stable operation of the melting furnace.
A melting-disposal method for solving such problems is disclosed in Japanese Unexamined Patent Publication No. 7-42,924 in which a melting furnace suppressing volatilization of low-boiling-point material is used. This method relates to an electrical-resistance heating-type melting furnace, in which electrodes immersed into the melt of the incineration residue are energized to heat the melt by the electric resistance heat and melt the incineration residue which is continuously fed.
FIG. 15 is a longitudinal cross-sectional view of such an electrical-resistance heating-type melting furnace. Reference numeral 30 represents a furnace main body, reference numeral 31 represents electrodes, reference numeral 50 represents a fed incineration residue, reference numeral 52 represents a molten slag layer formed by melting the incineration residue, and reference numeral 51 represents a molten salt layer formed by the melting of salts in the incineration residue. Also, reference numeral 33 represents an outlet of the exhaust gas and reference numeral 34 is an outlet of the melt. In this melting furnace, the molten salt layer 51 inside a partition wall 35 provided between the roof and the melting zone in the furnace main body is covered the fed incineration residue 50, and the molten salt layer 51 outside the partition wall 35 is covered with a powdery material having a lower specific gravity than that of the molten salt, such as carbon, to suppress volatilization of the low-boiling-point materials from the molten salt layer.
The incineration residue containing salts is a mixture of oxide components having melting points ranging from 1,400.degree. C. to 1,600.degree. C. and salt components having melting points ranging from 700 to 800.degree. C., whereas the incineration residue is heated to a high temperature region higher than the melting points of the oxide components during the furnace operation, at which all the components are melted. Therefore, salts having low melting points are heated to an excessively high temperature. Because the melting points of these salts range from 1,400.degree. C. to 1,600.degree. C. and their vapor pressures markedly increase at near 1,100.degree. C., the low-boiling-point materials are extremely volatile in the furnace.
In this regard, the prior art method cannot suppress the vapor pressure of the molten salt as a source of the low-boiling-point materials regardless of the use of a remedy in which the molten salt and the incineration residue are covered with a powdery material such as carbon, because such a remedy has merely an intention of collecting the molten salt with the incineration residue and carbon. Therefore, such a method is not effective against suppression of volatilization of the low-boiling-point materials.